Semi-conductor devices and methods of making same



May 6, 1958 s. M. CHRISTIAN 2,833,969

SEMI-CONDUCTOR DEVICES AND METHODS OF MAKING SAME Filed Deo. l; 1953 SEMI-CONDUCTOR DEVICES AND METHODS F MAKING SAME Schuyler M. Christian, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1.953,Serlal No. 395,382

9 Claims. (Cl. 317-.-235) This invention relates to novel semi-conductor devices and methods for making them, and more particularly to such devices comprising a semi-conductor body of novel composition.

It is known to make semi-conductor devices such as transistors comprising a .body of semi-conductive germanium or silicon. For the production of satisfactory transistors the resistivity of the semi-conductive body is relatively critical. For most transistor applications employing germanium a resistivity within the range of about 1-5 ohm-cm. is desired. In certain instances, however, such as high frequency transistors the resistivity of the body should be as low as about 0.1 ohm-cm. It has been A determined that intrinsically pure germanium and silicon have resistivities of approximately 60 ohm-cm. and 60,000 ohm-cm., respectively, at room temperature, and are not generally suitable for use in transistors.

Generally, conduction in a body is effected by charge carriers, electrons and holes, which are available in intn'nsically pure semi-conductive material in insucient numbers to provide satisfactory transistor operation. Further, transistor operation is believed to be largely dependent upon an excess of charge carriers of one type over charge carriers of opposite type, i. e., a differenceV between the number of available electrons and thenumber of holes in the semi-conductor body must exist. Intrinsically pure semi-conductive materials are believed to contain available electrons and holes in approximately equal numbers and, therefore, additional charge carriers of a selected typeA are ordinarily provided by doping the material. Doping is generally accomplished by adding a selected impurity material to a relatively pure semiconductive material.

It has not been found practical to add impurity materials uniformly throughout a body in the solid state. Such addition would depend principally upondilusion, which at any temperature below the melting point of the body is an impracticably slow process. Generally, impurities are added to a molten mass of the material and the molten mass is then frozen.-

A principal problem in thus producing a semi-conductive material having a desired resistivity concerns the uniformity of a body produced. The resistivity of a semiconductor body is inversely proportional `to the impurity concentration. This uniformity, therefore, depends primarily upon the distribution of impurities throughout the body. Due to the fact that most impurity substances are more soluble in molten than in solid germanium or silicon, rst frozen portions of a doped semi-conductor body generally contain less impurity material than do later frozen portions. In view of these ditiiculties a process called zone-levelling or zone-melting is commonly utilized.

The general theory of zone-levelling is discussed in an article by W. G. Pfann entitled, Principles of Zone- Melting published in the Journal of Metals, July 1952. The process, briefly, comprises causing a relatively narrow molten zone to traverse the length of an elongated body United' States Patent 2,833,969v Patented May 6, 1958 ice of a relatively pure semi-conductive material. An impurity material is placed initially in the molten zone and the segregation phenomenon is relied upon to distribute the impurity material relatively uniformly throughout the body as the molten zone traverses its length.

In the ca se of germanium, for example, the quantities of impurity materials to be added to such a molten zone in order to produce an ingot of germanium having a desired resistivity have in certain instances been determined. For example, it has previously been Ifound that au ingot comprising 3 ohm-cm. germanium may be produced by zonelevelling when an impurity concentration of about 2X10 of antimony is established in the molten zone.

Likewise, a concentration of about 105 of indium may be employed to yield a germanium ingot having a resistivity of about 3 ohm-cm. Such ingots are preferably produced in the form of a single crystal.

These proportions are relatively small, and even when working with a relatively large body of germanium such as one wherein a molten zone of about grams may be conveniently utilized, it is not readily practicable to measure the required quantity of doping material directly. It is instead usually preferred to dilute the doping material by dissolving it in additional germanium. For example, if the quantity of impurity material desired is about 0.2 mg. it may be provided by adding about 20 mg. of an alloy consisting of 99% germanium and 1% impurity. However even this quantity is relatively diicult to measure with a high degree of accuracy, and further, it is relatively difficult to provide an alloy of indium and germanium having the desired proportionswith a high degree of uniformity.

Impurity materials are classified generally into three categories. Impurities such as antimony, which when alloyed with germanium provide free electrons in the conduction energy band, are called n-type or donor impurities. Materials such as indium, which when -alloyed with germanium provide acceptor centers or holes in the valence energy band, are called p-type or acceptor impurities. Impurities such as lead, tin or silicon which do not affect the conductivity type of germanium are called neutral impurities. A

It is an object of the present invention to provide novel p-type semi-conductor materials suitable vfor use in mak,n

ing transistors.

Another object is to provide p-type semi-conductive germanium and silicon containing novel impurity materials.

Another object is to provide p-type semi-conductive germanium and silicon of novel composition.

Another object is to provide a novel method of making p-type semi-conductive germanium and silicon.

Another object is to provide semi-conductor devices including improved semi-conductive materials.

Still another object is to provide transistor devices comprising p-type semi-conductor bodies of novel composition.

It has now been discovered that transistors and similar devices, having desirable operating characteristics, can be made from single crystalline germanium or silicon containing certain small percentages of manganese. It has further been discovered that the segregation coefficient of manganese in germanium and in silicon is relatively small, of the order of 10-5 to 10. Therefore manganese is particularly advantageous when used to dope rgermanium and silicon since relatively large and easily weighed quantities may be employed. e

Unexpectedly manganese has now been found to produce p-type conductivity when dispersed in germanium and silicon. This is directly contrary to the previous beliefs that manganeseyielded n-type impurities, because it is to the right of the fourth column of the periodic table according to Mendeleet. It had previously been believed that manganese impurity atomsacted according to, their higher valences, 6 or `7, when dispersed in 'germanium or silicon. It has now been discoveredthat a lower valence, 'probably 3, is etective in determining the conductivity types of germanium and silicon doped with manganese. Previous eroneous observations may be explained by the fact that the manganese previously used for doping germanium and silicon was relatively impure and included certain n-type impurity-yielding materials. If these n-type materials had segregation coecients substantially larger than the coecient of manganese, they could readily dominate the doping process and thus determine the conductivity type of the semi-conductor. Since segregation coeicients often dier by factors of 1000 and more it may be readily seen how a relatively small impurity concentration in manganese may have a larger effect in a doping process than does the bulk of the manganese itself.

The invention will be more fully described with reference to the drawing of which:

Figure 1 is a series of curves showing the variations of the relative impurity concentrations of a series of sngle crystal ingots of germanium produced by the zonelevelling process.

Figures 2 and 3 are schematic, cross-sectional, elevational views of devices produced according to the instant invention.

Figure 1 illustrates one of the advantages of the instant invention as compared with previous practice. Curve A is an idealized curve that shows the variation of relative impurity concentration along the length of a p-type germanium crystal doped with indium and grown by the zone-levelling process. Curve B shows the relative impurity concentration of another p-type crystal grown inr exactly similar manner but doped with manganese dae'- cording to the instant invention. The crystal doped with manganese is relatively more uniform over its length than the crystal doped with indium., The resistivity of a semi-conductor varies inversely according to its impurity concentration. Thus the manganese-doped crystal is more uniform with respect to resistivity and a larger proportion of its length is useful to produce transistors. This improved uniformity of impurity distribution results from the fact that the segregation coeflicient of manganese is relatively much smaller than the coefficient of previously used p-type impurity materials.

The segregation coefficient of an impurity substance in germanium, for example, may be dened as the ratio of the concentration of the impurity substance on the solid side of the interface of a growing ingot to the concentration on the liquid side of the interface. (Concentration in solid+concentration in liquid.)

It has now been discovered that the segregation coeicient of manganese in germanium is about l0'6 and in silicon is about 2X 10-5. These coeicients may be compared to the coeicients for two other commonly used p-type impurities, i. e., indium which has a coetcient of about 1x10s and gallurn which has a coecient of about 10"1. The relatively small segregation coecient' of manganese not only provides a greater uniformity in a crystal doped with manganese than in previous crystals, but also permits the addition of relatively large and easily weighed quantities of manganese to the material in` process.

Technitium and rhenium are generally similar to manganese, but since they are relatively rare and not readily available commercially they are notdiscussed in detail herein.

A single crystal of germanium having a resistivity of about 3 ohm-cm. may be produced according to the instant invention by the zone-levelling process generally in accordance with the method described in the article by W. G. Pfann mentioned heretofore as follows;

.though this size is not critical.

A solid ingot of semi-conductive germanium of relatively high purity and having a resistivity of about 40-50 ohm-cm. is placed in an elongated crucible which may be of silica. Somewhat less pure germanium may be utilized but more uniform results are obtained if the resistivity is at least about 35 ohm-cm. The germanium ingot may conveniently be about 2 cm. x 2 cm. x 50 cm. long, al

The crucible may conveniently be of about the same size and shape as the germanium ingot although this also is not critical. About 920 mg. of manganese is placed at one end of the germanium. It is preferred to place the manganese within a small depression or cavity upon a surface of the germanium not adjacent the silica crucible in order to permit the manganese to diffuse evenly throughout the molten zone and to minimize any absorption or reaction of mariganese into or with the crucible.

A relatively small single crystal of germanium is placed in the crucible adjacent the manganese-bearing end of the ingot. This crystal acts as a seed to guide the growth of the entire ingot into a single crystal. If it is desired to produce a polycrystalline .manganese-doped ingot the seed crystal may be omitted.

The crucible bearing the germanium charge is placed within a silica tube within a zone-melting furnace. The silica tube is provided with means to maintain a nonoxidizing atmosphere such as dry hydrogen about the germanium during the process.

A zone about 4.5 cin. long is melted at the manganesebearing end of the charge and maintained in a molten state at about 960 to 1000 C. for about 5-15 minutes to permit the manganese to diffuse evenly throughout the zone and to insure complete melting of the germanium. This zone includes a portion of the seed crystal in order to insure growth of a single crystal structure as the germanium freezes. The molten zone is then caused to progress at a. uniform rate of about l mm. per minute from the seed end of the ingot to the far end. As the zone progresses, the solid polycrystalline ingot is slowly melted and a single crystal of germanium grows attached to the seed crystal. A relatively small proportion of the manganese initially placed in the molten zone is uniformly distributed along the length-of the grown crystal s except for the last frozen 4.5 cm. portion which comprises a relatively large proportion of the initially added manganese.

'Ihe variation of relative impurity concentration of the major portion of a crystal grown in this manner is shown in curve B of Figure 1 and may be compared with that of a crystal grown in similar manner but doped with indium as shown in curve A.

In the production of certain transistor devices, especially those designed for high frequency applications, it is often desired to provide a semi-conductive germanium body having a resistivity of about 1.0 ohm-cm. Such a material may be readily provided according to the practice of the instant invention by a method exactly similar to that described heretofore, except that about 2760 mg. of manganese are added to the molten zone. A relatively large portion of the length of a crystal grown by this method is su'iciently uniform to be useful in the production of transistor devices.

In a similar manner semi-conductive germanium may be produced having a resistivity of about 10 ohm-cm. A

such as may be desired for use in certain devices. The amount of manganese to be added to themolten zone in this case is about 276 mg., and the remainder of the process may be exactly as heretofore described.

The last grown portions of crystals or ingots produced in accordance with the instant invention, although not generally suitable for transistor production, are not Wasted but are utilized to make new ingots or crystals. Since these last portions include almost all of the manganese originally added to the ingots they may be conveniently used as the starting portions of new ingots or crystals, thus saving additional impurity measuring steps.

' nitude only, and are not exactmeasures.

assspes It should be noted that the advantages of the instant invention are not limited to the zone-levelling procese. Especially in the case of silicon, and to a lesser extent in the case of germanium, the practice of the invention is advantageous in other processes such as the vertical crystal pulling technique because of the relatively hig melting point and low volatility of manganese. l

In doping a molten zone to grow a crystal of manganese-doped germanium, the concentration of manganese established in the molten zone is critical. For example, to produce a crystal having a resistivity of about 3 ohmcm., it is known that the impurity concentration in the frozen crystal should be about 104; i. e., the crystal should include one elfective impurity center (manganese atom) for each a atoms of the pure crystal material. The segregation coeflicient of manganese is about l0, and therefore if the molten zone comprises an atomic manganese concentration of about l0"2 a crystal grown from the zone will have the desired resistivity. For example, in the process heretofore described a molten zone 4.5 cm. long and having a cross section 2 cm. square was described. This molten zone comprises about 100 grams of germanium. The atomic weights of germanium and manganese are about 72 and 55, respectively,` and therefore a quantity of about 920 mg. of manganese is utilized to provide a manganese concentration of about 10-2.

According to such simple, known relationships the quantity of manganese required in a zone-levelling process to produce a semi-conductor body having a desired resistivity may be roughly determined. The precise quantity is preferably determined empirically since many variables must be taken into account. For example, the apparent, or effective value of the segregation coet'ricient may be varied over relatively wide limits by varying such,-V

factors as the speed of crystal growth, the temperature of the melt, or the impurities present in the semi-conductive material immediately prior to the process.

In general, to providesemi-conductive germanium or silicon of the most generally useful resistivity, within the range of l to l0 ohm-cm., an atomic impurity concentration of about l0'I to 10-9 must be established in the material. In the practice of the instant invention a desired impurity concentration in the solid material may be provided by maintaining a concentration of manga, nese in the melt of about 10-2 to 10-4.

It should be understood that the concentrations described in the preceding paragraph are orders of mag- Exact gures are not given because of the relatively large number of variables that may affect the process.

It has been found that the purity of the manganese utilized in the practice of the invention is relatively critical. It is relatively important that the manganese used to dope semi-conductive materials according to the instant invention contain less than about 0.1 atomic percent of impurities that are n-type in germanium and have segregation coefficients of about l0-s or greater. As explained heretofore, impurities in the manganese otherwise may dominate the conductivity characteristic of manganese-doped material if they are present in sufficient quantities and have relatively large segregation coeicients in the semi-conductive material.

According to a preferred embodiment of the invention a transistor may be produced utilizing a wafer of germanium doped with manganese. The wafer may be cut by known techniques from a single crystal grown by the zone-levelling process.

A typical device utilizing manganese-doped germanium is illustrated in Figure 2. Figure 2 shows an alloy junction transistor comprising a base wafer 4 of manganesedoped germanium produced according to the instant inven'tion and having a resistivity of about 3 ohm-cm. The wafer may conveniently be about 0.25" x 0.25" x .006" thick. A pair of electrodes 7 and 8 of an alloy of 90% lead and 10% antimony are fused to opposite surfaces 5 and 6 respectively of the wafer. Within the wafer there are disposed two opposite p-n rectifying junctions 9 and 11. each adjacent one of the electrodes. Two electrical leads l2 and 14 makecontact with the respective electrodes. A'metal tab 16 is attached to the wafer by means of a non-rectifying solder connection 17. When utilized in a circuit the smaller electrode 7 may conveniently be employed as an emitter, the larger electrode 8 as a collector and the wafer 4 as a base.

The manner of operation of a device such as that described in the preceding paragraph is not' critical according to the present invention. In all ways that can presently be determined'the performance of a device produced in accordance with the instant invention is comparable to the performance of previous such devices that utilize p-type semi-conductive germanium doped with materials other than manganese. n

A device utilizing -a `body of relatively low resistivity germanium produced according to the invention is illustrated in Figure 3. Figure 3 shows a point contact transistor suitable for high frequency operation. This transistor comprises a base wafer 22 of manganese-doped germanium about .04" x .04" x .02" and having a resistivity of about l ohm-cm. An electrical lead 26 is bonded to one surface 23 of the wafer by means of 'a non-rectifying solder connection 28. Upon the opposite surface 25 of the wafer two closely spaced relatively hard, pointed metallic wires 30 and 32 lare pressed. The ends of the wires are sharpened to chisel points so that the areas of contact between the wafer and the wires are minimized. The ends of the wires contact the wafer at two points about .0005" apart. One of the wires may be advantageously employed in a circuit as an emitter electrode, the other wire as a collector electrode, and the lead 26 may conveniently serve as a base connection.

There have thus been described semi-conductive germanium of novel composition, methods of making it and devices utilizing such germanium.

What is claimed is.'

l. In a semi-conductor device comprising a semi-conductor body of a material selected from the class consisting of germanium and silicon and electrodes connected to said body, the improvement consisting of said body comprising a region of p-type semi-conductive material having an excess of p-type over n-type impurities, said excess being of the order of 101 to 10"" atomic percent, and said excess impurities being atoms of manganese.

2. The invention according to claim 1 in which said excess is of the order of 10 atomic percent, and said resistivity is about 2-5 ohm-cm.

3. In a transistor comprising -a plurality of spaced electrodes in contact with the surface of a semi-conductor body, the improvement consisting of said body having a region of p-type semi-conductive material, said material being selected from the class consisting of germanium and silicon having a resistivity of 1 to 10 ohm-cm. and having an excess of p-type over n-type impurities, said excess being of the order of 10-6 to 10-'I atomic percent, and said excess impurities being atoms of manganese.

4. In a transistor comprising a semi-conductor body of a material selected from the class consisting of germanium and silicon having adjacent regions of n-type and p-type conductivity,- respectively, a p-n rectifying junction disposed at the boundary between said regions, and an electrode attached to said n-type conductivity region, the improvement consisting of said body having a region of p-type semi-conductive material, said material having a resistivity of 1.0 to km-cm. and having an excess of p-type over n-type impurities, said excess being of the order of l0 to 10"? atomic percent, and said excess impurities being atoms of manganese. v

5. The invention according to claim 4 in which said semi-conductor body consists essentially of germanium.

6. A semi-conductor 'body of a material selected from the class consisting of germanium and silicon, said body 9. A body according to claim 6, said body having a being ofsubstantially single crystal structure comprisresistivity oil to 10 ohm-cm. ing an excess of p-type over-n-type impurities, said excess f being of the order of 10-5 to 1 0'l atomic percent, and References Cited in the tile of this patent said excess impurities 'being atoms of manganese.

7. A `body according to claim 6, said body consisting UNITED STATES PATENTS essentially of silicon. 

1. IN A SEMI-CONDUCTOR DEVICE COMPRISING A SEMI-CONDUCTOR BODY OF A MATERIAL SELECTED FROM THE CLASS CONSISTING OF GERMANIUM AND SILICON AND ELECTRODES CONNECTED TO SAID BODY, THE IMPROVEMENT CONSISTING OF SAID BODY COMPRISING A REGION OF P-TYPE SEMI-CONDUCTIVE MATERIAL 